Distinct evolution history of magmatic oxygen fugacity and its control on associated porphyry Cu deposits under subduction and collisional settings

Abstract

Porphyry deposits occur not only in subduction zones, but also in collisional belts worldwide. The magmatic oxygen fugacity (ƒo2) is an important element for the formation of porphyry, and influences the processes in source, migration, and precipitation of Cu and other chalcophile elements. Magmatic sulphur exists mainly as sulphate () at higher oxygen fugacity, which combines with chalcophile elements (e.g., Cu, Au) as ion complexes then formed the ore‐bearing magma. In contrast, magmatic sulphur exists mainly as sulphide (S2−), sulphide will precipitate easily. A magmatic event usually involves multiple intrusive activities. These porphyries have various ƒo2 evolutions, implying that they are formed in different formation environments or dynamic mechanisms. In a subduction setting, plates begin to dehydrate and then partial melting occurs in the early stage of subduction. Following the extensive dehydration, partial melting occurs as the plate diving deeper. As a result, the later magma is more oxidative than the earlier one. While under the collisional setting, porphyritic magma is likely related to the previous arc magmatism of subduction or juvenile mantle components in the thickened lower crust. The hydrous and metalliferous fertile magma formed. Then, reduction of water supply is the possible factor that leads to the gradually decrease of ƒo2. As a result of this study, metallogenic elements are usually developed in later porphyritic magma at a subduction environment, but in the early porphyritic magma at a collisional environment. This can provide guidance for prospecting of porphyry deposits in multi‐phases of porphyritic magma districts.